Note: Descriptions are shown in the official language in which they were submitted.
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PCT/DE2017/100490
METHOD AND DEVICE FOR PRODUCING HOLLOW MICROGLASS BEADS
The invention relates to a method and a device for producing hollow microglass
beads in the diameter range from 0.01 mm to 0.1 mm from molten glass, which
beads
can be used inter alio as a filler for lightweight building materials or as a
constituent part
of coatings, paints and plasters/renders.
The production of solid microglass beads in the diameter range up to 0.015 mm
is
known from DE 10 2008 025 767 Al or DE 197 21 571 Al, according to which
molten
glass particles are dispersed by means of a cutting wheel.
A comparable method for producing hollow glass beads is described in
WO 2015/110621 Al. In order to be able to produce hollow microglass beads with
diameters from 0.01 mm to 0.12 mm using this technology, very high cutting
wheel
speeds are necessary, wherein technical limits are encountered in the mounting
of the
cutting wheel (uneven running) and the cooling (wind formation). Consequently,
hollow
microglass beads in the required diameter range cannot be produced by this
method.
DD 261 592 Al describes a method for producing solid microglass beads in the
diameter range from 0.040 mm to 0.080 mm from molten high-index glass. The
molten
glass in the form of a glass strand of approximately 4 mm to 6 mm diameter
comes out
of a platinum melting vessel and is atomised to form glass particles using a
cold jet of
compressed air with a velocity of 100 m s-1 to 300 m s-1 and a pressure of 300
kPa to
700 kPa. It is a disadvantage that, during the atomisation of soda-lime
glasses, glass
filaments are produced instead of the required glass particles.
The documents US 2 334 578 A, US 2 600 936 A, US 2 730 841 A, US 2 947 115 A,
US 3 190 737 A, US 3 361 549 A, DE 1 019 806 A and also DE 1 285 107 A
describe how
cullet is ground, sifted and partially screened to the size of the solid
microglass beads to
be produced. The material is delivered to a temperature field in which, due to
the
surface tension, the individual glass particles take on a spherical shape
during their
passage through a heating zone. However, during the time-consuming grinding of
the
shards the grinding media and the mill are subject to substantial wear;
moreover, with
this method it is not possible to control the size of the glass beads.
DE 10 2007 002 904 Al discloses a method for producing hollow glass beads from
finely ground soda-lime glass and/or borosilicate glass by means of a heat
transfer
process (for example in a shaft furnace). As a result of the lowering of the
viscosity of
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the glass particles, the temperature rising according to the method results in
the
production of glass beads due to the surface tension. Furthermore, the high
temperature effects the gaseous emission of an added propellant. Consequently,
the
small solid beads grow to form larger hollow beads. Disadvantages are the
costly
crushing of the glass and the defective control of the hollow bead size, so
that
subsequent classification is necessary.
According to AT 175672 B, molten glass which runs out of a nozzle as a strand
is
dispersed by an intermittently acting hot air jet into glass particles which
assume a
spherical shape during the subsequent free fall. The intermittent hot air jet
is created by
a perforated rotating disc. Only comparatively large beads can be produced by
this
method.
Further methods for glass bead production are described in US 2 965 921 A,
US 3 150 947 A, US 3 294 511 A, US 3 074 257 A, US 3 133 805 A, AT 245181 B
and also
FR 1 417 414 A. With the methods referred to therein the fundamental problems
and
disadvantages, such as for example glass filament formation, low output,
complicated
atomisation systems, great fluctuation in the diameter of the microglass
beads, are not
prevented. The microglass beads must be subsequently cleaned of fibres by
additional,
extremely costly technological method steps. When liquid media are used,
additional
drying of the microglass beads is necessary.
The object of the invention is to provide a method and a device for producing
hollow microglass beads which makes it possible to manufacture the hollow
microglass
beads in a diameter range from 0.01 mm to 0.1 mm in a continuously operating
process
directly from molten glass while avoiding glass filament formation. The
dispersion range
of the diameter of the hollow beads produced according to the method should be
less
by comparison with currently known production methods.
According to the invention the production of the hollow microglass beads takes
place by atomisation of a molten glass strand by means of a hot gas to produce
glass
particles, wherein, during a passage through a heated rounding/expansion duct
following the atomisation, solid microglass beads are rounded and are
subsequently
expanded to form hollow microglass beads.
In a melting device, for example a platinum tank or a conventional melting
tank,
the glass is melted with a predetermined composition, wherein at least one
substance
which is gaseous in the range from 1100 C to 1500 C is contained in dissolved
form in
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the glass melt.
In the bottom region of the melting device there is located a discharge
opening,
through which the glass melt exits in the form of one or more glass strands.
A nozzle plate with a plurality of nozzles formed as conical through openings
is
preferably arranged on or inside the discharge opening, so that a plurality of
glass
strands spaced apart from one another are produced at the outlet of the glass
melt from
the melting device. The nozzle plate is preferably directly electrically
heated.
By means of a hot gas flowing out of a high-pressure hot gas nozzle, for
example a
natural gas/oxygen high- pressure burner, the molten glass strand or strands
is or are
atomised to form glass particles after the outlet from the melting device,
wherein the
glass particles produced have a more or less irregular configuration. The hot
gas flow is
preferably oriented at right angles to the glass strand or strands.
Due to the flowing hot gas the glass particles are subsequently blown directly
into
the immediately adjoining rounding/expansion duct oriented in the flow
direction.
During the passage through the rounding/expansion duct the rounding (spherical
shaping) of the glass particles to produce solid microglass beads takes place,
i.e. during
the heating the glass particles take on a spherical shape or are transformed
into beads
as a result of the surface tension.
In the course of the further passage, by suitable temperature control in the
rounding/expansion duct the expansion (inflation) of the solid microglass
beads into
hollow microglass beads takes place as a result of the degassing of the
dissolved gaseous
substance.
The rounding/expansion duct is operated in the temperature range from
usually 1100 C to 1500 C by the hot gas and possibly by additional heating
systems.
After the outlet from the rounding/expansion duct the hollow microglass beads
are cooled by means of cooling air and collected in solid form.
One of the advantages of the invention is that, due to the high gas velocity
and
the high gas temperature of the hot gas flowing out of the high-pressure hot
gas nozzle
onto the glass strand or strands, the formation of glass filaments is avoided.
By compliance with constant conditions, namely the gas temperature, the gas
velocity and the process temperature, a small dispersion range of the size of
the hollow
microglass beads is ensured which is in the diameter range from 0.02 mm to
0.05 mm.
Costly subsequent classifications of the hollow microglass beads are omitted
in fractions
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with a narrow diameter bandwidth.
The method makes it possible with continuous process management to produce
high-quality hollow microglass beads cost-effectively and in large quantities
per unit of
time. Expensive method steps, such as for example the mechanical comminution
of cold
glass and the cost-intensive heating until the rounding takes place, are
unnecessary.
At the outlet from the melting device the glass strands advantageously have a
diameter from 0.5 mm to 1.5 mm.
The viscosity of the glass melt exiting as a glass strand is preferably 0.5
dPa s
to 1.5 dPa s. With a given chemical composition of the glass melt, the setting
of this
viscosity range can take place by control of the melt temperature.
Furthermore, at the outlet from the melting device the glass strand or strands
is
or are subjected to a flow of the hot gas with a gas velocity in the range
from 300 m
to 1500 m s-1, preferably 500 m s' to 1000 m s-1. The temperature of the hot
gas is set
particularly suitably to a value between 1500 C and 2000 'C.
Soda-lime glasses or borosilicate glasses are preferably used for the method
according to the invention. The glass compositions for particularly suitable
soda-lime
glasses or borosilicate glasses are apparent from the details according to
Table 1.
Table 1: Preferred composition of the glasses for producing the hollow
microglass
beads
Soda-lime glass Borosilicate glass
Constituents Proportion by mass / % Proportion by mass! %
SiO2 60 - 64 65 - 74
Na2O 15 - 18 1 - 2
CaO 16 - 18 1.0 - 1.5
A1203 1.5 - 2.5 2 - 3
B203 1 - 6 12 - 16
SO3 0.6 - 0.8
As203 0.1 - 0.5
Sb203 0.1 - 0.5
BaO 1 - 2
ZrO2 4 - 5
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ZnO 2 - 4 1 - 4
It can be provided that the substance which is dissolved in the glass melt and
is
gaseous in the range from 1100 C to 1500 C is sulfur trioxide, oxygen,
nitrogen, sulfur
dioxide, carbon dioxide, arsenic oxide, antimony oxide or a mixture thereof.
In the case of sulfur trioxide (503) the preferred proportion by mass is in
the range
from 0.6 % to 0.8 %, wherein the proportion of sulfur trioxide can be
implemented for
example by an addition of sodium sulfate in the glass melt. Furthermore,
suitable
dissolved, gaseous substances are arsenic oxide (A5203) or antimony oxide
(Sb203)
having a proportion by mass in the range from 0.1 % to 0.5 %.
Particularly advantageously, the respective proportion by mass of the
dissolved
substance is chosen as follows:
sulfur trioxide (S03) 0.8 %
antimony oxide (5b203) 0.5 %
arsenic oxide (As203) 0.5 %
In an embodiment of the invention a transport gas is blown in axially into the
rounding/expansion duct by means of a transport gas nozzle (of a transport
burner). The
flow direction of the transport gas corresponds to the duct direction and the
injection
takes place below the region in which the glass particles enter the
rounding/expansion
duct. The transport gas serves to keep the glass particles, the solid
microglass beads as
well as the hollow microglass beads suspended during the passage through the
rounding/expansion duct and to assist the transport through the
rounding/expansion
duct. Furthermore, the transport gas can be used for heating the
rounding/expansion
duct.
The device for carrying out the method comprises the melting device with the
discharge opening arranged in the bottom region, on which or inside which the
nozzle
plate is mounted in such a way that the glass melt can exit exclusively from
the nozzles
in thin glass strands. The high-pressure hot gas nozzle is located immediately
below and
alongside the discharge opening and is oriented so that when the method is
being
carried out the hot gas flowing out of the high-pressure hot gas nozzle
impinges on the
glass strands (3.1) exiting from the nozzles.
The rounding/expansion duct is located in the flow direction of the hot gas
which,
during operation, flows out of the high-pressure hot gas nozzle after the
discharge
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opening.
Furthermore, for delivery of the cooling air the device has a cooling air
funnel
which adjoins the rounding/expansion duct, wherein the cooling air funnel and
also the
rounding/expansion duct are oriented in the flow direction of the hot gas. The
funnel
opening is facing the rounding/expansion duct. The funnel neck of the cooling
air funnel
forms a discharge duct for collecting the cooled hollow microglass beads.
The termination of the end region of the discharge duct arranged in the flow
direction can be formed by a cyclone precipitator or a rotary feeder, by means
of which
the hollow microglass beads are continuously conveyed out of the discharge
duct.
In one embodiment of the invention the nozzle plate has nozzles each having a
circular cross-section and having a diameter in the range from 1 mm to 3 mm.
This
makes it possible to produce the glass strands in the diameter range from 0.5
mm
to 1.5 mm which is particularly advantageous for the method.
Furthermore, it can be provided that the nozzles of the nozzle plate which are
spaced apart from one another are arranged in a line. The positioning of the
linear
nozzle arrangement in the device takes place transversely with respect to the
flow
direction of the hot gas.
In this embodiment the nozzle plate can have two symmetrically curved
reinforcing beads which extend in mirror image to one another along the
linearly
arranged nozzles. Heat-induced deformations or distortions of the nozzle plate
are
restricted by the reinforcing beads; a geometrically exact exit of the glass
strands from
the nozzle is guaranteed. The reinforcing beads can be formed for example in
sheet
metal components of the nozzle plate.
This nozzle plate is preferably made from a platinum material.
The invention is explained in greater detail below on the basis of embodiments
and with reference to the schematic drawings. In the drawings:
Figure 1 shows the device for carrying out the method for producing hollow
microglass beads, and
Figure 2 shows the nozzle plate with five nozzles in top view and in cross-
section.
According to a first exemplary embodiment according to Figure 1, soda-lime
glass
is melted with a proportion by mass of 0.8 % of sulfur trioxide in the melting
device 1, an
electrically heated platinum melting vessel, at 1450 C. By means of the
discharge
opening 1.2 in the bottom of the melting device 1, molten glass 3 enters
through the
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electrically heated nozzle plate 2 made of platinum with 20 linearly arranged
nozzles 2.1
with a respective diameter of 1.5 mm out of the melting device 1. The
viscosity of the
glass melt 3 is 0.5 dPa s. The exiting molten glass strands 3.1 with a
diameter of 0.7 mm
are atomised immediately after the exit from the nozzle 2.1 by the hot gas 14
from the
high-pressure hot gas nozzle 4 of an oxygen/natural gas high-pressure burner
to form
glass particles 3.2. In this case the hot gas flows at right angles against
the glass
strands 3.1 with a gas velocity of 600 m/s. Then the glass particles 3.2 enter
the
immediately adjoining rounding/expansion duct 6 which is made from refractory
material and is longitudinally heated by means of the transport gas 15 from
the
transport gas nozzle 5 of a transport gas burner.
The temperature in the rounding/expansion duct 6 is 1500 C. The solid
microglass
beads 3.2 initially formed from the glass particles 3.2 in the
rounding/expansion duct 6
then expand to form hollow microglass beads 3.4 and ultimately enter the
discharge
duct 9 made from stainless steel. Cooling air 7 is blown into this duct via
cooling air
funnels 8 for cooling the exhaust gases, and then exits again at the end of
the discharge
duct 9 as exhaust air 11 through the sieve 10. The sieve 10 prevents the exit
of the
hollow microglass beads 3.4. These are conveyed out of the discharge duct 9
through
the rotary feeder 12. The hollow microglass beads 3.4 have a diameter from
0.02 mm
to 0.05 mm.
In a second exemplary embodiment borosilicate glass with a proportion by mass
of 0.5 % antimony oxide in einem conventional melter at a melting temperature
of 1600 C. The molten glass 3 enters the feeder at a temperature of 1450 C
through an
electrically heated discharge opening 1.2 with a sieve insert to keep
refractory particles
away from the electrically heated nozzle plate 2 with 22 linearly arranged
nozzles 2.1
having a diameter in each case of 1.5 mm. The atomisation of the molten glass,
the
transport through the rounding/expansion duct 6 and the discharge correspond
to those
in the first exemplary embodiment. The diameter of the hollow microglass beads
3.4 is
in the range from 0.02 mm to 0.04 mm.
The nozzles 2.1 of the nozzle plate 2 according to Figure 2 exhibit above and
below the row of nozzles in each case a symmetrically curved reinforcing bead
2.2. The
reinforcing beads 2.2 are formed in the sheet metal components of the nozzle
plate 2.
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List of reference numerals used
1 melting device / crucible
1.1 insulation
1.2 discharge opening
2 nozzle plate
2.1 nozzle
2.2 reinforcing bead
3 glass melt
3.1 glass strand, molten
3.2 glass particle
3.3 solid microglass bead
3.4 hollow microglass bead
4 high-pressure hot gas nozzle
5 transport gas nozzle
6 rounding/expansion duct
7 cooling air
8 cooling air funnel
9 discharge duct
10 sieve
11 exhaust air
12 rotary feeder
13 discharge of the hollow microglass beads
14 hot gas
15 transport gas
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